In an analysis of the influence of amino acid deprivation in cultures of normal and transformed cells, Pardee postulated a "restriction point" in normal cells, a point in the cell cycle of normal cells which prevented progression through division. This restriction point was absent in transformed cells. These cells continued to progress through the cell cycle in spite of the deficiency, to die at a later time while in various states of the cycle. The restriction point of normal cells was further defined as a form of quiescence, with the deprived, normal cells considered to be at G(O) stste, an equilibrium point of the cell cycle removed from cell division. This absence of growth rendered normal cells less susceptible to cytotoxic agents and protected them from damage during chemotherapy. A large literature has developed on this point with the promise that amino acid deprivation may be an important adjunct to cancer chemotherapy. Warrington and others have applied this premise with the use of histidinol, which causes histidine deprivatin by blocking histidine activation for formation of histidyl-tRNA. Our unit has shown that uncharged transfer RNA inhibits the controlling enzyme of glycolysis, phosphofructokinase, and has presented observations from the literature which support the thesis that the restriction point control resides in the availability of the product of this enzyme, fructose-1,6-diphosphate. When phosphofructokinase is limiting for glycolysis, its inhibition quickly reduces the level of fructose-1,6-diphosphate, thus compromising several diverse metabolic pathways. Indeed, Bosca and Corredor reported that phosphofructokinase is not limiting in tumor cells, but is in normal cells. They evaluated the presence or absence of its limiting role by the magnitude of the ratio between the substrate and product of the enzyme, the fructose-6-phosphate fructose-1,6-diphosphate ratio. The broad ranges of the ratios found indicated the need for evaluating this parameter in cells of clinical importance at various stages of proliferation.